ST. PAUL, Minnesota (CNN) - Police on Tuesday night used tear gas to prevent demonstrators from trying to climb over a security fence around the site of the Republican National Convention, a police spokesman said.

Police in riot gear and wearing gas masks told the crowd that if it did not disperse, they would use “a chemical agent” to break it up.

By the time the announcement was completed, shells contain tear gas were apparently being released

Tom Walsh, public information officer for the St. Paul Police Department, said that police took action when a “subset of the group” which had marched to the the Xcel Center made clear its intentions to breach the security fence.

Police also said they used “distraction devices” against the crowd.

More than 200 people were arrested when demonstrators clashed with police on Monday, police said, but no one was arrested on Tuesday.